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Gold Linker Patch: Introduce the "retpoline" x86 mitigation technique for variant #2 of the speculative execution vulnerabilities disclosed today, specifically identified by CVE-2017-5715 and in some places called "spectre".

Sriraman Tallam via binutils binutils@sourceware.org
Mon Jan 8 18:29:00 GMT 2018 
On Mon, Jan 8, 2018 at 10:27 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Mon, Jan 8, 2018 at 10:22 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Mon, Jan 8, 2018 at 10:19 AM, Sriraman Tallam via binutils
>> <binutils@sourceware.org> wrote:
>>> Hi Cary,
>>>
>>> On Sun, Jan 7, 2018 at 3:14 PM, Cary Coutant <ccoutant@gmail.com> wrote:
>>>>> We aren't patching linkers just because we can. ;]
>>>>
>>>> Chandler, if I didn't know you personally, I'd take offense. Hmm,
>>>> maybe I took a wee bit of offense, even so. :-) Snark isn't going to
>>>> get you anywhere; it's more likely to close minds.
>>>>
>>>> The details of these vulnerabilities are out there now, and your
>>>> little circle is much bigger. That means you're now exposed to people
>>>> with different experiences and possibly more expertise. Even if you've
>>>> already been down certain paths and answered certain questions, you're
>>>> going to need to do it again for the rest of us. People on this thread
>>>> are asking reasonable questions, and if you want help and cooperation,
>>>> those questions deserve serious answers. Together, we may even come up
>>>> with better solutions.
>>>>
>>>> In particular, I'd like to know your answer to Alan's question about
>>>> the performance implications of deploying slow shared libraries where
>>>> not all applications need this mitigation, and the suggestion to just
>>>> compile secure apps statically. I'd like to know your answer to HJ's
>>>> suggestion to eliminate the PLT altogether (I have an answer to that,
>>>> but I'd like to know yours).
>>>
>>> Let me try to answer some of these questions since these were
>>> discussed and considered:
>>>
>>> * Plain Static linking is going to be a problem since it does not work
>>> with PIE + static.  We have investigated using musl libc to enable
>>> static linking + PIE and we had somebody who was successfully able to
>>> link the dynamic relocator within a statlic binary to make PIE work
>>> but it looks like it is going to be quite a while before this can be
>>> deployed.
>>> * We are looking at partially statically linking hot memops to remove
>>> the PLT + ifunc penalty for these calls.  I understand that we may
>>> still have to rebuild libc.so to use retpoline maybe.
>>> * Regarding what HJ said, unless I misunderstood, I believe he is
>>> referring to using fno-plt.  We considered that but the problem is the
>>> indirect jump still exists, but now at the call site.  The mitigation
>>> would still be necessary at the call site as it is still exposed to
>>> the attack.
>>
>> Not with GCC using -mindirect-branch=thunk -fno-plt.

Thanks!, not yet supported in LLVM.  Is this faster than the plain
indirect jump?  Any pointers to the patch discussions?

Thanks
Sri

>>
>
> [hjl@gnu-bdw-1 indirect-2]$ cat x.i
> void func (void);
>
> void
> bar (void)
> {
>   func ();
> }
> [hjl@gnu-bdw-1 indirect-2]$ make x.o
> /export/build/gnu/gcc/build-x86_64-linux/gcc/xgcc
> -B/export/build/gnu/gcc/build-x86_64-linux/gcc/ -O2 -g
> -mindirect-branch=thunk -fpie -fno-plt -c x.i
> [hjl@gnu-bdw-1 indirect-2]$ objdump -dwr x.o
>
> x.o:     file format elf64-x86-64
>
>
> Disassembly of section .text:
>
> 0000000000000000 <bar>:
>    0: 48 8b 05 00 00 00 00 mov    0x0(%rip),%rax        # 7 <bar+0x7>
> 3: R_X86_64_REX_GOTPCRELX func-0x4
>    7: e9 00 00 00 00        jmpq   c <bar+0xc> 8: R_X86_64_PC32
> __x86.indirect_thunk.rax-0x4
>
> Disassembly of section .text.__x86.indirect_thunk.rax:
>
> 0000000000000000 <__x86.indirect_thunk.rax>:
>    0: e8 05 00 00 00        callq  a <__x86.indirect_thunk.rax+0xa>
>    5: 0f ae e8              lfence
>    8: eb fb                jmp    5 <__x86.indirect_thunk.rax+0x5>
>    a: 48 89 04 24          mov    %rax,(%rsp)
>    e: c3                    retq
> [hjl@gnu-bdw-1 indirect-2]$
>
>
>
> --
> H.J.
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